Method of forming fixed images

ABSTRACT

A method of forming fixed images includes charging a photoconductor; exposing the photoconductor to light; developing an electrostatic latent image, such that a toner is applied to the electrostatic latent image formed on a transfer film moving in partially close contact with the photoconductor to form a visible image; transferring the formed visible image onto a recording medium such as a recording paper; and fixing the transferred visible image onto the recording medium. The toner is an encapsulated toner, the photoconductor is a heat-resistant photoconductor, and the transfer process and the fixing process are simultaneously carried out on the surface of the heat-resistant photoconductor covered with the transfer film onto the preheated recording medium.

TECHNICAL FIELD

The present invention relates to a method of forming fixed images usedfor plain paper copying machines, laser printers, plain paperfacsimiles, etc. More particularly, the present invention relates to amethod of forming images in which transfer and fixing are simultaneouslycarried out on the surface of the heat-resistant photoconductor coveredwith a transfer film using a recording medium whose surface is heated inadvance.

BACKGROUND ART

Conventionally, when images are formed with copying machines, laser beamprinters, etc., the Carlson Method has been generally used (U.S. Pat.Nos. 2,221,776, 2,297,691 and 2,357,809, "Electrophotography," p22-p41,R. M. Shaffert, 1965, The Focal Press).

FIG. 2 shows a schematic view of an apparatus used in a conventionalmethod of forming fixed images. In the conventional method, after anelectrostatic latent image formed on a photoconductor by optical meansis developed in a developing process, it is transferred to a recordingmedium such as a recording paper in a transfer process and then fixedinto the final image generally with heat and pressure in a fixingprocess. As the photoconductor is repeatedly used, a cleaning device isprovided for cleaning the residual toner after the transfer process withits rotation.

In the conventional method of forming fixed images, however, theprocesses from the formation of the electrostatic latent image up to itsfixing onto the recording medium are time consuming, which makes theapparatus used therein not only complicated but also large. In addition,since the transfer efficiency of the toner is poor in the transferprocess, it poses such problems as extra labor needed for the disposalof the toner collected by cleaning the residual toner, and pollution dueto the scattering of the toner in and out of the apparatus.

Therefore, a method of simultaneously conducting transferring and fixinghas been proposed (U.S. Pat. No. 4,448,872). In this method, since thetransferring and fixing are simultaneously carried out by pressing thetoner image developed on the dielectric drum to the recording medium,the simplification of the apparatus can be surely achieved. However,since only pressure is applied at fixing, the fixing ability is poor,and little improvement is achieved in transfer efficiency.

The fixing of the toner should be generally conducted at a hightemperature due to the high melting temperature of the toner, therebyrequiring an apparatus with high thermal efficiency. The fixing processusually works independently, and is carried out at such a hightemperature of around 200° C. Accordingly, expensive heat-resistantmaterials such as heat-resistant resins, heat-resistant rubbers, etc.have to be provided in the periphery of the fixing device.

In addition, when the fixing is carried out at a high temperature, it issubject to problems such as curling and jamming of the paper, etc.Therefore, taking into consideration of the radiation from theapparatus, a device highly capable of radiating heat is in demand.Further, if the fixing requires a high temperature, it takes more timeto reach the set temperature so that a quick printing becomesimpossible. In such a case, therefore, this method is unsuitable fordevices such as a facsimile which requires quick printings.

Further, in view of solving the problems, there has been proposed amethod of forming fixed images, wherein the transfer and fixing processis simultaneously carried out by adhering the toner onto a transferfilm, which rotates while keeping it partly in close contact with aphotoconductor to form a toner image, and putting the recording paperand the transfer film between a pressure roller and a heat rollerprovided away from the photoconductor (Japanese Patent Laid-Open No.197884/1990).

According to this method, however, when the transfer film is woundaround the pressure roller, one of the pair of rollers for transfer andfixing, and a heat roller is arranged on the outside of the transferfilm, the recording medium, which is heated from the reverse side, suchas paper, etc. having insulating effects has a poor thermal efficiency,and thereby a sufficient heat required for fixing cannot be supplied tothe toner. Therefore, problems arise in that fixing becomesinsufficient. On the other hand, when the transfer film is wound aroundthe heat roller and the pressure roller is arranged on the outside ofthe transfer film, the heating material is arranged in the inside of thefilm belt, causing problems in radiation from the internal portion ofthe film belt. When the heating material is arranged inside the filmbelt, the radiation conditions are likely to be insufficient, therebycausing deterioration in sensitivity and decrease in durability of thephotoconductor due to heat. In addition, since the fixing is carried outthrough the film, problems may arise in the delaying of the transmissionof heat, thereby presumably demanding a higher fixing temperature forthe heat roller.

Further, in the conventional method of forming fixed images, however,through the processes from the formation of the electrostatic latentimage up to its fixing onto the recording medium, the temperature of theheating material of the fixing device has to remain at a very high level(usually around 200° C.) and further a relatively high pressure isrequired (usually between 2.0 and 6.0 kg/cm). On the other hand, sinceboth the photoconductor and the developing device have to be maintainedat around room temperature, a considerable distance has to be maintainedbetween the fixing device and the developing device, which necessitatesto make the machine larger. In addition, it is necessary to force theremoval of the generated heat from the system, but the noise produced bythe forced radiation device is not negligible.

As for solving these problems, a device for carrying out low temperaturefixing using a cold pressing method (Japanese Patent Laid-Open No.159174/1984) is known. In this reference, however, although the fixingtemperature is low, the nip pressure has to be elevated normally to notless than 4 kg/cm in this method, making the machine heavier. Moreover,it poses problems in the gloss of the images, deformation of the papercopy sheets and insufficient fixing strength. As for a fixing device forfixing images at such a low nip pressure of less than 4 kg/cm, a heatroller method is known, for example, but it has been pointed out thatthe fixing temperature needs to be maintained at not less than 120° C.

Under these circumstances, the development of a fixing device that canfix images at a low temperature and at a low nip pressure is highlydesired, but it has not yet been developed. Further, as regards tonersto be indispensably used for the image formation, since they have beenconfined to those made from a thermoplastic resin dispersed withadditives such as coloring agents, charge control agents, releasingagents, etc., and pulverized, there have been limitations on themolecular weight, the softening point of the thermoplastic resin for usein the toner from the aspect of storage stability, thereby posinglimitations on the further pursuit of low temperature fixing.

From these standpoints, the development of a novel method of formingfixed images as well as a matching toner therefor is in demand.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a novel method offorming fixed images, which solves the various problems as mentionedabove, thereby namely achieving good transfer efficiency, no disposedtoner and miniaturization of the image forming device.

Therefore, in view of solving the above-mentioned problems, the presentinventors have investigated a toner shell material which is fragile toheat at a low temperature. As a result, they have found that a thermallydissociating encapsulated toner produced by interfacial polymerizationmelts at a temperature of not more than 120° C., and they have furtherinvestigated the image formation method using this encapsulated tonerand have thus developed the present invention.

More particularly, in view of solving the above problems, the method offorming fixed images of the present invention comprises charging aphotoconductor; exposing the photoconductor to light; developing anelectrostatic latent image whereby a toner is applied to theelectrostatic latent image formed on the transfer film moving inpartially close contact with the photoconductor to form a visible image;transferring the formed visible image onto a recording medium such as arecording paper; and fixing the transferred visible image onto arecording medium, wherein the toner is an encapsulated toner, thephotoconductor is a heat-resistant photoconductor, and the transferprocess and the fixing process are simultaneously carried out on thesurface of the heat-resistant photoconductor covered with a transferfilm onto a preheated recording medium.

The heat-resistant photoconductor is a silicon photoconductor, a zincoxide photoconductor dispersed in resin or an organic photoconductor,and the photoconductor comprises a binder having a glass transitionpoint of not less than 100° C.

In the present invention, the transfer film is a photopermeant film inaccordance with the wavelength of the light source.

In addition, the transfer process and the fixing process aresimultaneously carried at a position between the transfer film and oneroller or one belt.

Further, the preheating temperature of the recording medium is at arange of between not less than 50° C. and not more than 160° C.

According to the present invention, the visible image formed on thetransfer film moving in partially close contact with the photoconductordrum in the developing process is simultaneously transferred and fixedto the recording medium whose surface is heated in advance. Therefore,the transfer and fixing process can be remarkably simplified. Also,since substantially all of the toner in the developing process istransferred and fixed, the disposed toner remaining untransferred doesnot take place, making it unnecessary to leave extra space for thedisposed toner. Accordingly, the device can be remarkably miniaturized.Also, the radiator can be made much smaller due to its low fixingtemperature, thereby achieving the miniaturization of an image-formingapparatus. In addition, since an independent transfer process is notrequired, an adjustment of electric resistance for the recording mediumsuch as recording paper is not also required. By using a photoconductorhaving a good heat resistance, the durability of the photoconductorbecomes longer, thereby remarkably increasing its reliability, and thephotoconductor can be miniaturized.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are notlimitative of the present invention, and wherein:

FIG. 1 is a schematic view of an apparatus used in the method of formingfixed images as defined by the present invention;

FIG. 2 is a schematic view of an apparatus used for conventional methodsof forming fixed images;

FIG. 3 is a schematic view showing the charging process in the method asdefined by the present invention;

FIG. 4 is a schematic view showing the exposing process in the method asdefined by the present invention;

FIG. 5 is a schematic view showing the developing process in the methodas defined by the present invention; and

FIG. 6 is a schematic view showing the transfer and fixing process inthe method as defined by the present invention.

The reference numerals in FIGS. 1 through 6 denote the followingelements:

Element 1 is a photoconductor, element 1a a photoconductive layer,element 1b a conductive supporter, element 2 an exposure device, element3 a developer device, element 3a a rotating sleeve, element n a heater,element 5a a pressure roller, element 5b a transfer device, element 6 arecording medium (a recording paper, etc.), element 7 a charger, element8 a cleaner device, element 8a a toner collecting box, element 9 acharge eraser, element 10 a toner, element 16 a holding roller, andelement 17 a transfer film.

BEST MODE FOR CARRYING OUT THE INVENTION

The method of forming fixed images of the present invention are detailedbelow, .referring to the drawings.

FIG. 1 is a schematic view of an apparatus used in the method of formingfixed images as defined by the present invention. Element 1 is aheat-resistant photoconductor. For photoconductors, those practicallyused are photoconductors of selenium, silicon, organic groups, etc.However, in the present invention, since the photoconductor is exposedto a considerable amount of heat, the silicon photoconductors, the zincoxide resin-dispersed photoconductors and the organic photoconductorshaving good heat resistance are preferred.

A silicon photoconductor is composed of amorphous silicon or siliconcarbide (Japanese Patent Laid-Open No. 86341/1979), and for example, thep-type photoconductor to which boron atoms are doped to the amorphoussilicon or the n-type photoconductor to which phosphorus atoms are dopedto the amorphous silicon can be used.

As zinc oxide resin-dispersed photoconductors, those having aphotoconductive layer comprising zinc oxide fine particles, sensitizerdyes and binders can be used (U.S. Pat. No. 2,952,536). From theviewpoint of not only in the sensitivity but also in the chargeabilityof the photoconductor, the zinc oxide fine particles preferably have aparticle size of 0.1 to 1 μm. The sensitizer dyes are appropriatelychosen in accordance with the wavelength of the light source in theexposure device. Examples thereof include xanthene dyes such as RoseBengal, etc., triphenylmethane dyes such as Crystal Violet, etc.,thiazine dyes such as Methylene Blue, etc. and cyanine dyes.

The organic photoconductors are single- or multi-layered photoconductorhaving a photoconductive layer comprising materials capable ofgenerating charges and transporting charges and binders, on theconductive supporter can be used. Examples of the charge generationmaterials include perylene pigments, condensed ring quinone pigments,phthalocyanine pigments, bisazo pigments, trisazo pigments, squaryliumpigments, etc., with preference given to the perylene pigments andphthalocyanine pigments. Examples of the charge transport materialsinclude hydrazone derivatives, pyrazoline derivatives, oxadiazolederivatives, arylamine derivatives, styrile derivatives, etc., withpreference given to the arylamine derivatives.

The binders which can be preferably used for the heat-resistantphotoconductor in the present invention have glass transition points ofnot less than 100° C. Typical examples thereof include condensationpolymers such as polycarbonates, polyarylates, polyesters, polyamides,etc., addition polymers such as polymethacrylate, styrenemethacrylatecopolymer, polyacetal, etc. and thermosetting resins such as epoxyresins, phenol resins, silicone resins, urethane resins, urea resins,etc. When the above binders are used, those having a glass transitionpoint of less than 100° C. are undesirable because the adhesion of thetoner and blurring of the latent image take place.

Any of the above photoconductors can be used as the heat-resistantphotoconductors for the present invention, as long as itsphotoconductive layer has a glass transition point of normally not lessthan 100° C. to meet the requirement in heat resistance.

Element 7 is a charger located opposite to the photoconductor 1. Thecharging means is not particularly restricted, and any of, for instance,a corona charger, a brush charger, a roller charger, etc. can be used.

Element 2 is an exposure device located opposite to the photoconductor 1for forming electrostatic latent images on the photoconductor surface.For an exposure device 2, light sources such as laser beams, LED or ELarrays, etc. are used in combination with an image-forming opticalsystem. Alternatively, a device based on optical systems projecting areflected light of a document generally provided in the copying machinecan be used.

Element 3 is a developer device located opposite to the photoconductor 1for making visible the electrostatic latent image formed on thephotoconductor with the toner. For a developer device, any of thecommonly used two-component magnetic brush developer device, theone-component magnetic brush developer device, the one-componentnon-magnetic developer device, etc. can be used.

As shown in FIG. 1, in the method of the present invention, a transferfilm 17 which moves in partially close contact with a heat-resistantphotoconductor is used to form an electrostatic latent image by adheringthe toner on the transfer film. It is necessary for the transfer film 17to have good heat resistance, since it is exposed to heat transmittedfrom the recording-medium which is preheated to a temperature of 50° C.to 160° C. It also should have a certain level of an insulating propertyas well as photopermeability in accordance with the wavelength emittedin the exposing process. The transfer film is not particularlyrestricted as long as it has a good photopermeability in accordance withthe wavelength of such a light source. Examples of such transfer filmsinclude those made of PVA film, PET film, polymethylpentene film,cellophane, polycarbonate film, ethylene-vinyl alcohol copolymer film,etc. In addition, taking into consideration of the lines of electricforces generated at the time of developing, the thickness of thetransfer film 17 is preferably not more than 200 μm, and taking intoconsideration of the tensile strength as well as the easiness inhandling, it is preferably not less than 10 μm. The transfer film 17 isstretched with, for instance, the photoconductor 1 and the holdingroller 16. The number of the holding rollers is not particularlyrestricted.

Element 4 is a heater, and element 5a is a pressure roller, and theheater 4 is disposed just before the point where the photoconductor 1which is in close contact with the transfer film 17 contacts thepressure roller 5a, so that the preheated recording medium can beconveyed to the pressure roller. In addition, although the distancebetween the photoconductor and the heater is preferably kept as littleas possible for the purpose of preventing the decrease in the surfacetemperature of the paper, the heater 4 is preferably set at a properdistance from the photoconductor so as not to cause thermal effects orthermal deformation on the photoconductor. The heater is normallyarranged at a distance of about 2 to 10 cm away from the point where thephotoconductor contacts the pressure roller. In addition, in order toincrease preheating efficiency, the heater is preferably arranged nearlyin parallel with the upper portion of the conveying route for therecording paper. The heater 4 is a device for preheating the surface ofthe recording medium such as a recording paper, wherein the surfacecomes in contact with the toner. As long as it is a device capable ofheating the surface of the recording medium up to 160° C., any type ofheat source can be used for the heater 4. Heating materials of theheater 4 include, for example, a hot plate, a quartz heater, a flashheater, a heating belt, a heater element, etc., with preference given tothe quartz heater and the heater element. The pressure roller 5a is ameans for pressure-welding the preheated recording medium onto thesurface of the photoconductor covered with the transfer film 17. In anordinary fixing device, it is necessary to use heat-resistant siliconerubbers, etc. in order to carry out fixing at a high temperature.However, in the present invention, it is not required to use thepressure roller having a particularly high heat resistance, since thepressure roller in contact with the reverse side of the preheatedrecording medium is not directly heated. Therefore, as long as thematerials for the pressure roller are elastic bodies having a good heatresistance at not less than 150° C., there are no limitations on itsmaterials, and any of the ordinary inexpensive elastic materialsincluding, for instance, heat-resistant polyurethane resins, acrylicresins, nitrile resins and non-conjugated diene terpolymer resins suchas EPDM can be used. In addition, since the nip pressure of the pressureroller is usually 0.1 to 4.0 kg/cm, preferably 0.2 to 2 kg/cm, thedurability thereof becomes longer. Incidentally, in the presentinvention, a belt may be used for a similar means in the place of thepressure roller.

After the transfer and fixing process, the cleaner device 8 such as acleaning web for removing trace amounts of the toner remaining on thetransfer film is arranged opposite to the holding roller 16.

The photoconductor 1, the pressure roller 5a and the holding roller 16rotate at a constant peripheral speed in the direction shown in FIG. 1by a specified driving means not illustrated in the figure. As a result,the transfer film 17 moves in the direction shown by an arrow in thefigure by a frictional force caused between the photoconductor 1 and theholding roller 16. The transfer film comes in close contact with thesurface of the photoconductor after carrying out the charging processand before entering the exposing process, and the transfer film isremoved from the surface of the photoconductor after the transfer andfixing process. On the other hand, a recording paper 6 used as arecording medium is conveyed in the manner shown in FIG. 1, which afterpassing the pressure roller 5a is discharged out of the system by apaper discharging means not illustrated in the figure. In thisconnection, the conveying speed of the recording medium and the heatingtemperature of the heater is so regulated that the preheatingtemperature for the recording medium is maintained within thepredetermined temperature ranges.

Examples of the thermally dissociating encapsulated toners which ispreferably used in the present invention will be described below, butthe present invention is not restricted to these alone.

The toner used in the present invention is a thermally dissociatingencapsulated toner. The encapsulated toner according to the presentinvention comprises a heat-fusible core containing at least a coloringagent and a shell formed thereon so as to cover the surface of the corematerial. In the present invention, the thermally dissociatingencapsulated toner means a toner which comprises a shell whose structureis fragile to heat, and a core material which can be fixed at a lowtemperature by pressure. More particularly, the shell structure changeswith heat, and at the point when pressure is applied, the core materialis discharged to effect the fixing of the toner. Depending on the rawmaterials and production methods, a large variety of encapsulated tonersare conceivable, and as long as they are within the range of therequired thermal properties, there are no limitations on what productionprocess or materials are used. Specifically, those having thermalproperties capable of melting the toner on the recording medium heatedin advance at a temperature range of between 50° C. and 160° C. and offixing the toner by pressure of a pressure roller can be properlychosen. In general, the fixing temperature of the toner to the recordingmedium is in the range of between 40° C. and 120° C.

For example, the toner used in the present invention is an encapsulatedtoner produced by an interfacial polymerization method or a spray-dryingmethod. In the interfacial polymerization method, a core materialsolution or dispersion is dispersed in a water in oil or oil in watertype emulsion system, while at the same time shell material monomers (A)are collected around the surfaces, where in the next method, monomers(A) and monomers (B) react. In the spray-drying method, after the corematerial is dispersed in a non-aqueous solution of polymer orpolymer-emulsion, the dispersed liquid is spray-dried. In the presentinvention, either method can be used for the production of theencapsulated toner. In the case of using the interfacial polymerizationmethod, it not only has the merit of an easy function separation for thecore material and shell material but also is capable of producing auniform toner in an aqueous state. Moreover, substances of low softeningpoints can be used for the core material in the interfacialpolymerization method, making it particularly suitable from the aspectof fixing ability. Accordingly, in the present invention, the thermallydissociating encapsulated toner produced by the interfacialpolymerization method among others is particularly preferred.

For shell materials, styrene resins (Japanese Patent Laid-Open No.80407/1973), polyamide resins (Japanese Patent Laid-Open No.66948/1983), epoxy resins (Japanese Patent Laid-Open No. 148066/1984),polyurethane resins (Japanese Patent Laid-Open No. 179860/1982),polyurea resins (Japanese Patent Laid-Open No. 150262/1987) and manyothers have been proposed. And as substances fixible under heat andpressure contained in the core material, thermoplastic resins havingglass transition points (Tg) of between and 50° C. such as polyesterresins, polyamide resins, polyester-polyamide resins, and vinyl resinscan be used.

As compared to the thermal properties of the core material, thestructure and the thermal properties of the shell material concernthemselves remarkably with the fixing ability of the entire toner. Sincea particular polyurethane resin among the above-mentioned resins for theshell materials is thermally dissociating, having excellent storagestability and fixing ability at a low temperature, it is an extremelyfavorable material for the method of forming fixed images of the presentinvention. As principal components of such a shell material, resinsobtainable from the reaction between an isocyanate compound and/orisothiocyanate compound and compounds containing a phenolic hydroxygroup and/or a thiol group are preferably used (EPO453857A).

The thermally dissociating encapsulated toner suitably used in thepresent invention can be produced by any known methods such asinterfacial polymerization, etc., and this encapsulated toner iscomposed of a heat-fusible core material containing at least a coloringagent and a shell formed thereon so as to cover the surface of the corematerial, wherein the main components of the shell are a resin preparedby reacting:

(A) an isocyanate and/or isothiocyanate compound comprising:

(1) 0 to 30 mol % of a monovalent isocyanate and/or isothiocyanatecompounds, and

(2) 100 to 70 mol % of at least a divalent isocyanate and/orisothiocyanate compounds with

(B) an active hydrogen compound comprising:

(3) 0 to 30 mol % of a compound having one active hydrogen atom reactivewith isocyanate and/or isothiocyanate groups and

(4) 100 to 70 mol % of a compound having at least two active hydrogenatoms reactive with the isocyanate and/or isothiocyanate groups

at a molar ratio of the component (A) to the component (B) of between1:1 and 1:20, and wherein at least 30% of all of the linkages formedfrom the isocyanate or isothiocyanate groups are thermally dissociatinglinkages.

According to the present invention, the thermally dissociating linkageis preferably one formed by the reaction between a phenolic hydroxyland/or thiol group and an isocyanate and/or isothiocyanate group.

The resins to be used as core materials of the encapsulated toneraccording to the present invention are thermoplastic resins having glasstransition points (Tg) of 10° to 50° C., and such encapsulated toner ofthe present invention having a softening point of 80° to 150° C. can beused. Since the toner used in the method of the present invention is notsubject to charging in the transfer process, not only insulatingencapsulated toners but also conductive encapsulated toners can be used.

Next, the individual processes of the method of forming fixed images bythe present invention having the above-mentioned construction will bedescribed.

FIG. 3 shows a charging process, FIG. 4 an exposing process, FIG. 5 adeveloping process and FIG. 6 a transfer and fixing process.

In the charging process, as shown in FIG. 3, a specified charge isuniformly supplied, e.g. by the corona charger 7 to the photoconductorsurface. A photoconductor sensitive to a positive charge is taken herefor an example, and the surface of the conductive supporter 1b is coatedwith the photoconductive layer 1a to form the photoconductor 1. A highvoltage is applied by the corona charger 7 to the photoconductive layer1a, thereby positively charging the surface of the photoconductive layer1a.

In the exposing process, as shown in FIG. 2, a light from the exposuredevice 2 is irradiated to the surface of the photoconductor covered withthe transfer film 17, so that a leakage of charges occurs only in theexposed parts and form an electrostatic latent image on thephotoconductive layer 1a.

In the developing process, as shown in FIG. 5, the tonertriboelectrically charged inside the developer device is transported bythe rotating sleeve 3a, and developed onto the transfer film inproportion to the charge on the photoconductor surface. The developingprocess is an assortment of normal development in which a reverselypolarized toner adheres to the charges by the Coulomb's force and ofreverse development in which the toner adheres to the charges lost dueto exposure to the light. The development process in the presentinvention applies to either method, but the case of the normaldevelopment is illustrated in FIG. 5.

In the transfer and fixing process, transfer and fixing aresimultaneously carried out on the surface of the photoconductor coveredwith the transfer film. As shown in FIG. 6, the visible image formed byapplying the toner to adhere to a latent image on the transfer film isconveyed. At the same time, a recording medium 6 such as a recordingpaper preheated by a heater 4 is pressure-welded on the transfer film bypressing the reverse side of the recording medium by a pressure roller5a so as to synchronize with the initial end of the image, and therebythe visible image is simultaneously transferred and fixed onto therecording medium 6. In other words, when the toner adhered to the latentimage formed on the surface of the heat-resistant photoconductor coveredwith the transfer film is pressure-welded to the recording medium, thedeformation of the shell structure of the encapsulated toner due to theheat held in the recording medium takes place at the same time with thedischarging of the core-material in the encapsulated toner due topressure of the pressure roller. As stated above, the transfer processand the fixing process are carried out simultaneously at a positionbetween the transfer film and one roller (pressure roller 5a).Alternatively, a belt can be used in the place of the roller, therebycarrying out transfer and fixing at a position between a transfer filmand one belt. When the temperature applied to the surface of therecording medium by the heater 4 is too high, the recording paper tendsto curl, and when it is too low, sufficient fixing of the toner cannotbe achieved, making record preservation difficult. Therefore, thesurface of the recording medium is usually heated to a temperature ofbetween 50° C. and 160° C., preferably between 50° C. and 120° C.

In the present invention, since substantially all of the toner istransferred to the recording medium, a toner collecting device is notrequired. Incidentally, although trace amounts of the toner may remainon the surface of the transfer film 17 after the transferring of thetoner to the recording medium 6, this toner can be removed bypressure-welding the transfer film with such devices as a cleaning webarranged opposite to the holding roller 16, making it possible torepeatedly use the transfer film.

Further, when the transfer and fixing process is completed, afterremoving the transfer film 17 from the photoconductor, the chargesremaining on the photoconductor are neutralized by a charge eraser 9such as a charge erasing lamp arranged opposite to the photoconductor 1,so that the photoconductor 1 is reused for the charging process.

In addition, the present invention is not confined to theabove-mentioned embodiments, and specifications of the kinds ofindividual apparatus, processes etc. can be revised based on theprinciples of the present invention.

By using the method of forming fixed images of the present invention,the following effects can be obtained:

(1) Since the photoconductor is not directly in contact with themagnetic brush of the developer device, the cleaning blade, etc., thephotoconductor can be protected, and thereby the durability becomeslonger and the freedom in design becomes larger for the photoconductor.

(2) Since the photoconductor has a durability against heat, thedurability thereof becomes longer.

(3) Since the fixing is carried out at a low temperature by using atoner having good fixing ability and by only preheating the recordingmedium, the fixing takes place by only using a pressure roller, therebymaking the apparatus extremely compact.

(4) Since the heating temperature is low and the heat from the surfaceof the recording medium does not directly contact the pressure roller, ahigh heat resistance is not required for the pressure roller.Accordingly, inexpensive elastic members can be used as the materials ofthe pressure roller, and the duration of the roller becomes long.

(5) Since substantially all of the toner is transferred to the recordingmedium in the transfer and fixing process, little loss of the tonertakes place, thereby causing substantially no toner to be dischargedfrom the apparatus. Accordingly, the toner collecting box and thecleaning process can be simplified, thus making it possible to achievelow cost and miniaturization in the overall apparatus.

(6) Since the toner for the low-temperature fixing is used, thetemperature of the heating body in the fixing device can be set low withonly a small rise in the temperature in the system, thereby making itpossible to miniaturize the forced radiation device.

(7) Since the surface of the recording medium is heated in a preheatingprocess, a cardboard paper is also applicable for the method of thepresent invention.

(8) Since the fixing takes place without going through an electrostaticprocess, the conductive toners can also be used, and an electrostaticinducing-type development capable of applying low voltage can also beused.

The present invention is hereinafter described in more detail by meansof the following working examples, but the present invention is notlimited by them.

Production Example of Encapsulated Toner

To a mixture comprising 70.0 parts by weight of styrene, 30.0 parts byweight of 2-ethylhexyl acrylate and 1.0 part by weight ofdivinylbenzene, 10.0 parts by weight of carbon black "#44" (manufacturedby Mitsubishi Chemical Industries, Ltd.), 4.0 parts by weight of2,2'-azobisisobutyronitrile, 9.5 parts by weight of 4,4'-diphenylmethanediisocyanate "Millionate MT" (manufactured by Nippon PolyurethaneIndustry Co., Ltd.) are added. The obtained mixture is introduced intoan attritor (manufactured by Mitsui Miike Kakoki) and dispersed at 10°C. for 5 hours to give a polymerizable composition. This composition isadded to 800 g of a 4% by weight aqueous colloidal solution oftricalcium phosphate which has been preliminarily prepared in a 2-literseparable glass flask, so as to give a concentration of 30% by weight.The obtained mixture is emulsified and dispersed with a TK homomixer(manufactured by Tokushu Kika Kogyo) at 5° C. and a rotational speed of10000 rpm for 2 minutes. A four-necked glass cap is set on the flask,and a reflux condenser, a thermometer, a dropping funnel fitted with anitrogen inlet tube and a stainless steel stirring rod are set thereon.The resulting flask is placed on an electric mantle heater. A solutionof 22.0 g of resorcinol, 3.6 g of diethyl malonate and 0.5 g of1,4-diazabicyclo [2.2.2] octane in 40 g of ion-exchanged water isprepared, and the resulting mixture is dropped into the flask in aperiod of 30 minutes through the dropping funnel while stirring.Thereafter, the contents are heated to 80° C. and reacted for 10 hoursin a nitrogen atmosphere while stirring. After cooling the reactionmixture, it is dissolved into 10%-aqueous hydrochloric acid. Theresulting mixture is filtered and the obtained solid is washed withwater, dried under a reduced pressure of 20 mmHg at 45° C. for 12 hoursand classified with an air classifier to give the encapsulated tonerwith an average particle size of 9 μm having a shell made of a resinhaving a thermally dissociating urethane linkage. The glass transitionpoint assignable to the resin contained in the core material is 30.2°C., and its softening point is 130.0° C.

Production Example of Reference Toner

To 100 parts by weight of a polyester resin (Bisphenol-type polyesterresin; softening point: 135° C.; Tg: 65° C.), 7 parts by weight ofcarbon black (manufactured by Mitsubishi Kasei Ltd., MA8), 3 parts byweight of a polypropylene wax (Sanyo Kasei Ltd., Biscol 660P), and 2parts by weight of a charge control agent (Orient Kagaku KabushikiKaisha, Bontron N-01) are mixed, and the resulting mixture is kneaded bya pressurized kneader. After cooling the obtained mixture, it ispulverized with a pulverizing mill and then classified with a classifierto obtain a toner having a particle distribution range of 5 to 25 μm andan average particle size of 10 μm. To 1 kg of the toner, 5 g ofcolloidal silica (Nihon Aerozil Ltd.: R972) is externally added toobtain a surface-treated reference toner.

TEST EXAMPLE 1

50 g of the toner obtained in Production Example of Encapsulated Toneris blended together with 1 kg of a commercially available coated ferritecarrier by using a V-type blender to obtain a developer 1. The obtaineddeveloper 1 is used to carry out copying by using a modified apparatusof a commercially available copying machine as schematically shown inFIG. 1. Specifically, a pigment sensitized zinc oxide photoconductordispersed in resin is used as a photoconductor, a polycarbonate filmmanufactured by Gunze Limited having a thickness of 0.15 mm is used as atransfer film, and a quartz heater is used as a heater and arranged at adistance of 5 cm away from the point where the photoconductor contactsthe pressure roller, arranged substantially parallel to the upperportion of the conveying route for the recording paper. In addition, byvarying the heating temperature and the conveying velocity, thetemperature on the surface of the recording paper is properly adjustedso as to preheat the paper surface to a temperature of between 60° C.and 160° C. The pressure roller used in the transfer and fixing is madeof silicone rubber having a roller diameter of 30 mm φ, and transfer andfixing are carried out at a nip pressure of 1.5 kg/cm and a peripheralspeed of 80 mm/sec.

As a result, the lowest fixing temperature of the paper surface is 120°C., and substantially no melting of the toner to the surface of thephotoconductor covered with the transfer film is observed at atemperature of between 100° C. and 160° C.

On the other hand, the toner obtained by the Production Example ofReference Toner is blended with a commercially available coated ferritecarrier to prepare a developer 2. Copying is carried out in the samemanner as above using the modified apparatus. As a result, the lowestfixing temperature of the paper surface is 140° C.

The lowest fixing temperature for the toner is the temperature of thepaper surface at which the fixing rate of the toner exceeds 70%. Thisfixing rate of the toner is determined by placing a load of 500 g on asand-containing rubber eraser having a bottom area of 15 mm×7.5 mm whichcontacts the fixed toner image, placing the loaded eraser on a fixedtoner image obtained in the fixing device, moving the loaded eraser onthe image backward and forward five times, measuring the opticalreflective density of the eraser-treated image with a reflectivedensitometer manufactured by Macbeth Co., and then calculating thefixing rate from this density value and a density value before theeraser treatment using the following equation. ##EQU1##

TEST EXAMPLE 2

The developer 1 obtained in Test Example 1 is used to carry out copyingby using a modified apparatus of a commercially available copyingmachine as schematically shown in FIG. 1. Specifically, an organicphotoconductor comprising a charge generation layer containingphthalocyanine dye and a charge transport layer containing hydrazone isused as a photoconductor, a polyethylene terephthalate film manufacturedby Toray Industries, Inc. having a thickness of 0.15 mm is used as atransfer film, and a heating sheet is used as a heater and arranged at adistance of 5 cm away from the point where the photoconductor contactsthe pressure roller. In addition, the temperature on the paper surfaceof the recording paper is properly adjusted so as to preheat the papersurface to a temperature of between 60° C. and 160° C. The pressureroller used in the transfer and fixing is made of EPDM having a rollerdiameter of 50 mm φ, and the developer 1 obtained in Test Example 1 isused to carry out a continuous copying test for 50,000 sheets byadjusting to a nip pressure of 1.0 kg/cm and a peripheral speed of 160mm/sec. As a result, under the condition in which the fixing temperatureis 120° C., the fixing is good, and substantially no curling, jamming,etc. of the paper are observed. Also, substantially no disposed toner isobserved.

On the other hand, the developer 2 obtained in Test Example 1 is used tocarry out continuous copying in the same manner as above. As a result,the formed image was deteriorated due to the offset effect on thetransfer film and the pressure roller after copying 5,000 sheets.

From these test examples, it is confirmed that by utilizing the methodof forming fixed images according to the present invention using athermally dissociating encapsulated toner, the lowest fixing temperaturecan be remarkably lowered, thereby resulting in no curling or jamming ofthe paper sheets fed to the copying machine.

The present invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

We claim:
 1. A method of forming fixed images, comprising:charging aheat-resistant photoconductor; covering a surface of said heat-resistantphotoconductor with a photopermeable transfer film; exposing saidtransfer film and said heat-resistant photoconductor to light, therebyforming electrostatic latent images on the surface of said transfer filmand said heat-resistant photoconductor; developing said electrostaticlatent images on the surface of said transfer film into a visible imageby applying an encapsulated toner to said transfer film in the area ofsaid electrostatic latent image; simultaneously transferring saidvisible image onto a preheated recording medium and fixing thetransferred visible image onto the recording medium by contacting saidrecording medium with the surface of said transfer film while saidtransfer film is in contact with the surface of said heat-resistantphotoconductor.
 2. The method according to claim 1, wherein saidheat-resistant photoconductor is selected from the group consisting of asilicon photoconductor, a zinc oxide photoconductor dispersed in resin,and an organic photoconductor, and wherein said photoconductor comprisesa tinder having a glass transition point of not less than 100°.
 3. Themethod according to claim 1, wherein said transfer process and saidfixing process are simultaneously carried out at a position between thetransfer film and one roller or one belt.
 4. The method according toclaim 1, wherein said recording medium is preheated to a temperature ofnot less than 50° and not more than 160°.
 5. The method according toclaim 1, wherein said encapsulated toner is a thermally dissociatingencapsulated toner which comprises a heat-fusible core materialcontaining at least a coloring agent and a shell formed thereon so as tocover the surface of the core material, wherein the main component ofthe shell is a resin prepared by reacting:(A) an isocyanate and/orisothiocyanate compound comprising:(1) 0 to 30 mol % of monovalentisocyanate and/or isothiocyanate compounds, and (2) 100 to 70 mol % ofat least divalent isocyanate and/or isothiocyanate compounds with (B) anactive hydrogen compound comprising:(3) 0 to 30 mol % of a compoundhaving one active hydrogen atom reactive with the isocyanate and/orisothiocyanate groups and (4) 100 to 70 mol % of a compound having atleast two active hydrogen atoms reactive with the isocyanate and/orisothiocyanate groupsat a molar ratio of the component (A) to thecomponent (B) of between 1:1 and 1:20, and wherein at least 30% of allof the linkages formed from the isocyanate or isothiocyanate groups arethermally dissociating linkages.
 6. The method according to claim 5,wherein said thermally dissociating linkage is a linkage derived fromreacting phenolic hydroxyl and/or thiol groups with the isocyanateand/or isothiocyanate groups.
 7. The method according to claim 5,wherein said heat-fusible core material comprises a thermoplastic resinas its main component, whose glass transition point is 10° C. to 50° C.8. The method according to claim 5, wherein the softening point of saidthermally dissociating encapsulated toner is 80° C. to 150° C.
 9. Themethod according to claim 1, wherein said recording medium is recordingpaper.